This invention relates in general to drive line assemblies suitable for transferring power in a vehicle, and in particular to an improved drive line apparatus for transferring torque from a rotating tubular driveshaft to a universal joint assembly.
Metallic torque transmitting shafts and similar components are widely used for many different applications. In particular, metal torque transmitting shafts are frequently used in vehicular drive trains including axle shafts, yoke shafts and the like. In use, these shafts can be subjected to relatively large torque loads imposed on them by the vehicle engine to move the vehicle. It is desirable for these torque-beating members to be as strong as possible while still being light weight. Typically, driveshaft members are hollow to provide the maximum strength for the weight involved. It is known that driveshafts must be rotated at speeds below their critical speed to avoid resonance which leads to self destruction. The critical speed for any given driveshaft is a function of the density, modulus and geometry of the material in the driveshaft. Generally, the smaller the driveshaft diameter, the lower the critical speed of the driveshaft. Therefore it is desirable for the driveshaft to be at least as large in diameter as a specified minimum size for a particular driveshaft length and composition of material.
Vehicle driveshafts usually are adapted with universal joint assemblies for connection to other rotating drive line elements. The universal joints help accommodate differences in angular alignment between two rotating elements, such as a transmission output shaft and a driveshaft tube. Universal joints further enable a small amount of relative movement between the driveshaft and an adjacent rotating part, such as a transmission output shaft or an axle assembly input shaft. In a typical vehicle driveline assembly, the driveshaft tube is connected at each of its ends to a tube yoke which connects to a universal joint assembly. The universal joint assembly usually consists a journal cross and four bearing assemblies. Typically, assembly involves securing the cross member into place with a bearing strap and a plurality of lug bolts. An important requirement for the assembly of the driveline apparatus is gaining access to various driveline elements for the insertion of tools necessary to complete the assembly. In particular, it is important to be able to reach the lug bolts during the assembly of the universal joint. Power tooling is used, and the tooling must be accurately aligned to obtain the proper torque on the lug bolts.
Past attempts to lower the weight of vehicle driveshafts have resulted in replacing traditional two-piece steel driveshafts with lighter weight one-piece aluminum alloy driveshafts. This reduces the driveshaft weight, but, because it is longer, requires a larger diameter driveshaft for the same critical speed. The critical speed of the driveshaft can be increased by covering the driveshaft with a high strength coating, such as a resin matrix reinforced with graphite fibers, but this increases the manufacturing cost. It would be advantageous if there could be developed a driveline assembly which would enable the substitution of lighter weight aluminum alloys for the traditional steel tubes, and yet not require the use of expensive reinforcing coatings. Any solution to the problem would have to include the requisite access for the tooling needed to assemble the universal joints at the ends of the driveline tube.
Another problem associated with drive line assemblies is the cost of manufacturing various parts making up the drive line assembly. Elements of the universal joint, for example, usually are costly to manufacture. Using light weight materials, such as aluminum alloys, for drive line elements still fails to significantly reduce the manufacturing costs of the driveline element. As an example, tube yokes made of aluminum alloys require forming to an initial shape and size, followed by machining the tube seat to the desired tolerances. Typically the forming to the initial shape is accomplished by casting or forging a body of metal into the general shape of the part. Elimination of the requirement for machining the tube seats would substantially reduce manufacturing costs of the tube yoke. As is well known, machining costs include the costs of the machining assets as well as maintenance costs and the labor required to operate the machines. It would be advantageous if a method could be developed for making tube yokes for drive line assemblies, where the tube seat requires no machining.